The invention is concerned with a headbox for a paper machine for making paper from a web of paper pulp, with the headbox having an adjustable lower lip.
A headbox for a paper machine should adjust the pulp density and fiber orientation profile of the paper pulp suspension, at the latest, before the suspension passes through the discharge slit of the headbox, so that the pulp density and fiber orientation profiles of the paper web correspond to the desired requirements over the entire width of the web, meaning, as a rule, that they are constant.
When operating a paper machine, there are many perturbing factors which hinder the achievement of the two above requirements. These perturbing factors include, for example, temperature and pressure fluctuations, manufacturing tolerances, and defects in the design or adjustment of the paper machine for the production process after the paper pulp is discharged from the headbox.
In order to influence these factors, the following state of the art has become known. German publication DE 35 14 554 proposes to change the pulp density locally; that is, to adjust the pulp density at certain points, depending on demand. However, it is not described how this should be accomplished.
German publication DE 40 19 593 A1 recommends that, upon deviation of the pulp density profile of the paper web at a certain point of the web width, the concentration C.sub.M of the respective section flow, and thus that of the flow leaving the respective mixer, should be changed correspondingly. In order to achieve this, the ratio of the amounts of control flows Q.sub.N /Q.sub.L, introduced to the mixture are changed. However, in the case of valves of the usual construction, it is difficult to avoid deviation of the section flow Q.sub.M leaving the mixer from the required value in an uncontrolled and unwanted manner.
In addition, it is known from German publication DE-OS 35 38 466 that a change of the volume flow of a section leads to influencing the fiber orientation angle in the discharge section of the headbox. If a section flow deviates from the required value in an uncontrolled manner, the fiber orientation will also change in an uncontrolled manner.
Furthermore, it is known from German publications DE 29 42 966 and DE-OS 35 35 849 that one can change the width of the discharge slit, for example, with threaded spindles for horizontally swinging or bending the upper lip. As a result, the throughput of the suspension can be altered locally. However, at the same time, the flow direction is also influenced locally, and thus the fiber orientation is affected. Namely, at the narrow parts of the discharge slit, the fibers will be disposed in a different flow direction than at the other parts of the discharge slit. This means that, although the consistency can be made uniform over the width of the headbox by this method of control, called displacement control, the originally good fiber orientation is destroyed.